1. Field of the Invention
The present invention relates to a substrate coated with a transparent film, a coating liquid for forming the transparent film, and a display device employing the substrate coated with the transparent film. More specifically, the present invention relates to a substrate coated with a transparent film, which has high strength, and is excellent in antireflection property, anti-static property, electromagnetic wave-shielding property, durability, water-resistance, chemical resistance, and in particular excellent in scratch resistance. The present invention relates also to a coating liquid suitable for the transparent film formation, and a display device having a front face plate constructed of the substrate coated with the transparent film.
2. Description of the Prior Art
For prevention of reflection at the surface of a substrate such as a glass plate, and a plastic sheet, an antireflection film is formed thereon. For example, a film of a low-reflectance material such as magnesium fluoride is formed on the glass or plastic sheet by vapor deposition, CVD, or a like method. However, such methods are costly.
In another method of formation of antireflection film, a coating liquid containing fine silica particles is applied on a glass surface to form a film with uniform surface roughness originated from fine silica particles. This method intends to prevent reflection of light by reducing normal reflection by making the reflection irregular on the irregular surface formed by fine silica particles, or this method intends to prevent reflection of light by an air layer in interspace between the fine particles. However, with this method, it is not easy to fix the particles onto the substrate surface or to form a single layer film thereof on the surface of the substrate and to control the surface reflectance.
For further improvement, the applicant of the present invention discloses use of a transparent coating film constituted of a composite oxide particles in which porous core particle coated with silica and a matrix. This transparent film has a low reflectance, and useful as a surface coat constituent for low reflectance of a substrate such as low reflectance glass and a low reflectance sheet or film (Japanese Patent Publication No. 7-133105).
Furthermore, in JPA 10-40834, it is described that the cathode ray tube has a layer containing mainly SiO2 as well as silicon materials and/or ZrO2 provided on a conductive layer containing conductive particles such as silver. It is also described that a coating layer containing mainly SiO2 as well as alkoxysilane containing the fluoroalkyl group is formed on the conductive particulate layer and the upper and lower coating layers are fired at the same time, in JPA 10-40834. By using alkoxysilane containing fluoroalkyl groups, the water tightness and chemical resistance of the film are enhanced to provide a cathode ray tube with a reflection preventing film which are effective in preventing AEF (alternating electric field) with high brightness and low surface resistance in JPA 10-40834.
The aforementioned transparent films, however, are not sufficient in scratch resistance, being liable to be scratched at the surface to become inferior in the transparency or the antireflection property of substrate.
The transparent substrate for the display panel of a cathode ray tube, a fluorescent display tube, or a liquid crystal display is conventionally coated with an anti-static film for prevention of electrification of the transparent substrate surface. The surface of this anti-static coating film may further be coated with the transparent film.
As the anti-static coating film, for example, the film having a surface resistance of about 102 to 1010 Ω/□ is known.
As an aside, it is known that the display devices like a cathode ray tube emit an electromagnetic wave. Therefore, it is known that an electroconductive film having a low surface resistance of about 102 to 104 Ω/□ is formed on the surface of the display panel of the cathode ray display tube or the like for shielding the electromagnetic wave and the electromagnetic field generated by the emission thereof in addition to the aforementioned anti-static function.
An example of the above-mentioned anti-static coating film is an electroconductive coating film formed by application of an electroconductive film-forming liquid containing the fine particles of electroconductive metal oxide such as ITO on the surface of the substrate. Another example of the electroconductive coating film of a low surface resistance for electromagnetic wave shielding is a coating film containing fine metal particulate formed on the surface of the substrate by application of an electroconductive film-forming liquid containing fine particles of electroconductive metal like Ag.
However, in the aforementioned electroconductive coating films formed on substrate, the fine metal particles contained therein can be oxidized, can grow to larger particles by metal ionization, or can be corroded. Thereby, the electroconductivity or light transmittance of the coating film may be decreased, which lowers the reliability of the display apparatus. Further, the electroconductive oxide particles and fine metal particles contained in the electroconductive coating film have high refractivity, which causes reflection of light, disadvantageously.
The disadvantages can be overcome by additional formation of a transparent coating film having a lower refractivity on the electroconductive film to prevent the reflection and to protect the electroconductive film.
The conventional transparent film which is coated with an electroconductive coating film containing the fine particles of an electroconductive metal oxide such as ITO has a low reflectivity of about 1% in the center wavelength portion around 500 nm to 600 nm (bottom reflectivity) of the visible light (wavelength region: 400 nm to 700 nm). However, the reflectivity is higher at the wavelength regions near 400 nm and near 700 nm. Therefore, the luminous reflection (average reflectivity over the entire visible light region) should be decreased as well as the bottom reflectivity (average reflectivity around the wavelength from 500 to 600 nm).
On the other hand, when a conventional transparent coating film containing a matrix composed of silica or the like is formed on the electroconductive film surface, the density of the electroconductive film is liable to become nonuniform owing to the difference in the shrinkage degree between the transparent film and the electroconductive film. As a consequence, failure of electrical contact between the electroconductive fine particles may result and insufficient overall electroconductivity of the film may develop.
Furthermore, in the heat treatment, the transparent coating film is not sufficiently densified and is porous so as to form cracks and voids, which may permit penetration of moisture and chemicals such as acids and alkalis, all of which are disadvantages.
An acid or alkali which penetrates into the transparent coating film may react with the surface of the substrate to lower the refractivity, or may react with the fine particles of the metal or the like in the formed electroconductive coating film, when it is employed, to lower the chemical resistance of the coating film and to decrease the anti-static property and electromagnetic wave shielding effect of the electroconductive film, disadvantageously.
In the case where the transparent film is formed on the surface of the electroconductive film containing the fine metal particles, although the bottom reflectivity is as low as 0.2%, the reflectivities near 400 nm and near 700 nm are high, and the luminous reflectivity is in the range of about 0.5 to 1%. This makes the visual feeling of the image reflection (mirror reflection) stronger, and the coloration of the reflected light cannot readily be suppressed. Therefore, the transparent film should further be improved in the anti-reflection property.
The inventors of the present invention, after comprehensive investigation on the low reflectivity film to be formed on the electroconductive coating film, found a transparent film comprising a matrix containing a silicone having a fluorine-substituted alkyl group, and an inorganic compound particles constituted of a shell and a porous matter or cavity enclosed therein. This transparent coating film has a sufficiently low refractive index, a low shrinkage property, and hydorphobicity, and adheres well to the substrate and the transparent electroconductive layer, having high film strength and high scratch resistance. The substrate coated with such a transparent film is excellent in durability, water resistance, chemical resistance, and anti-reflection. The transparent film formed on the electroconductive film surface gives excellent anti-static property and excellent luminous reflection factor, preventing the mirror reflection, and the coloration of the reflection. Thereby, a display device can be made which has excellent display performance. Thus the present invention has been achieved.